The present disclosure relates to a power amplifier. A Doherty amplifier is known as a power amplifier with high efficiency. The Doherty amplifier is generally constituted by a carrier amplifier that operates regardless of a power level of an input signal, and by a peak amplifier that is turned off when the power level of the input signal is low, and that is turned on when the power level of the input signal is high, the carrier amplifier and the peak amplifier being connected in parallel. When the power level of the input signal is high, the carrier amplifier operates in a state of maintaining saturation at a saturation output power level. As a result, the Doherty amplifier is able to increase efficiency in comparison with an ordinary power amplifier.
Regarding a modified example of the Doherty amplifier, Masaya Iwamoto, Aracely Williams, Pin-Fan Chen, Andre G. Metzger, Lawrence E. Larson, Peter M. Asbeck, “An Extended Doherty Amplifier With High Efficiency Over a Wide Power Range” IEEE TRANSACTIONS ON MICROWAVE THEORY AND TECHNIQUES, VOL. 49, NO. 12, 2001, p. 2472-2479, for example, discloses a Doherty amplifier capable of providing a wider range of an output power level where efficiency is increased in comparison with that obtained by a general Doherty amplifier. Furthermore, Japanese Unexamined Patent Application Publication No. 2016-19228 discloses a Doherty amplifier constituted without using a λ/4 line that is used in the general Doherty amplifier.
According to the Doherty amplifier disclosed in the above-cited paper, when a combined saturation output level of the carrier amplifier and the peak amplifier is assumed to be a reference, a range up to reaching a point at which the carrier amplifier comes into a saturation state (i.e., a range where the carrier amplifier operates with high efficiency) (the range being also referred to as a “back-off” hereinafter) can be made relatively wide. However, because the relevant Doherty amplifier includes a λ/4 line, it is not adapted for a device, such as a cellular phone, for which downsizing is highly demanded. On the other hand, the Doherty amplifier disclosed in Japanese Unexamined Patent Application Publication No. 2016-19228 can reduce a circuit size because of not including the λ/4 line. However, the back-off of the relevant Doherty amplifier is relatively small. Accordingly, for a signal having a high PAPR (Peak to Average Power Ratio), for example, the carrier amplifier does not operate in a highly-efficient state, and there is a limitation in improving the efficiency.